1. Field of the Invention
The present invention relates to turbochargers, or more particularly to multivariable dual stage series turbochargers having at least two degrees of freedom.
2. Description of the Related Art
Multi-stage, in particular dual, series turbocharging is an emerging and important technology. However, the control of multivariable turbo systems pose some problems. A dual stage turbocharger has two degrees of freedom whereas a single stage turbo has only one degree of freedom that is relatively straightforward to set.
The present invention concerns multistage series turbochargers, as opposed to parallel turbochargers, from a feedback control point of view. The invention can use the turbocharger to control to setpoints on various variables rather than the prior art switching or ad hoc strategies. For example, on a single stage turbocharger with variable nozzle vanes (VNT) actuators on the turbine, a relatively common activity is to manipulate the VNT vanes to control the boost pressure, as measured by a manifold air pressure sensor, to achieve a desired setpoint. This invention uses a series turbocharger with VNT vane actuators on both the high pressure (HP) turbine and the low pressure (LP) turbine and exploiting the two degrees of freedom to control the engine. The dual stage turbocharger is a mechanical device that brings an additional degree of freedom to the turbocharger and therefore it is possible to control to two setpoints. Usually this will be boost pressure plus one other variable.
U.S. Pat. No. 3,941,104 discloses the mechanical design of a series turbocharger. U.S. Pat. No. 4,299,090 discloses parallel turbochargers where the main control is by using valves to shut down one turbo in certain circumstances. No real attempt at optimal coordination of the two turbos is made. U.S. Pat. No. 5,063,744 describes a dual series turbocharger with a bypass valve on the HP turbine where the actuators are ramped at two different rates. U.S. Pat. No. 5,186,005 follows a procedure of shutting down one turbocharger in certain cases as defined by measured air flow, pressure etc. This approach uses an on/off logic for switching between one turbocharger operation to two turbocharger operation. U.S. Pat. No. 6,311,493 shows two turbochargers in series with two actuator options, namely a bypass valve on a high-pressure turbine, or bypass valves on high-pressure turbine and a low-pressure turbine. The control uses a feed forward schedule rather than feedback control to the setpoint. U.S. Pat. No. 6,550,247 uses multiple parallel turbochargers where a specific VNT turbine vane actuator is constrained to move between discrete steps. The disclosure proposes a sequential movement to minimize disruption into engine. U.S. Pat. No. 6,801,846 uses two turbochargers in series where an actuator is a bypass valve around one of the turbines. The control portion of computes a setpoint for this valve based on measured engine parameters such as speed, pressure etc.
By contrast, the present invention continually moves the turbocharger actuators to continually work to drive sensor measurements to their setpoint targets. The need of dual-stage turbocharging is to coordinate small and large turbochargers in an effort to obtain the benefits of both. Traditional single stage turbocharging must achieve a tradeoff between conflicting objectives. A fast response in boost pressure (pi) requires a small turbocharger with low inertia while high boost pressures require a large turbocharger. Fuel economy requires a large turbocharger. A dual turbocharged system requires a careful scheduling and coordination of turbocharger action in order to ensure obtaining the benefits while avoiding the disadvantages. This invention presents a control strategy to provide coordinated multi-stage turbocharging using a variety of actuator and sensor configurations.